In principle, there's no difference between the value of a link regardless of where it appears in a page (I mean, apart from logic specific to each crawler). Update: as Mike Hudson showed in his answer, different sections of a page are valued differently, but I can't say for sure how well-recognized the "blog" pattern is - between many more forms a website can take. A crawler specialized in blogs, however, is likely to perform well in this regard.
Anyway, in the specific case of blog comments (or other tools that accept lots of anonymous user content), most softwares automatically add the rel="nofollow" attribute to each link that appears in a comment (mostly to prevent abuse). As a result, those links do not influence the rank of the target site when search engines crawl it.
Quoting the Wikipedia article linked above:
nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of search engine spam, thereby improving the quality of search engine results and preventing spamdexing from occurring.